This invention relates to a differential gear mechanism for motor vehicles, in particular agricultural tractors, intended to drive rotatively two axle shafts of a motor vehicle live axle.
As is known, in the field of motor vehicles and specifically of four-wheel drive agricultural tractors, torque must be transmitted to both front axle wheels even when under poor and different traction conditions.
Conventional differential gear mechanisms using a planetary or the like system, are designed to apportion the drive torque according to a predetermined ratio; the front axle wheels can therefore be driven at different rotational speeds related to each other.
This differential gear design is only effective where the live axle wheels are subjected to equal traction conditions, because if either slips while the other grips, the speed of the slipping wheel may continue to increase, whereas the speed of the gripping wheel may decrease continuosly until the motor vehicle is brought to a stop.
Several solutions to the problem have been proposed whereby the differential gear can be locked using either positive or friction clutches, either in a fully automatic or manually operated fashion, to provide full or partial locking of the differential gear.
Some examples of differential gear mechanisms are described in U.S. Pat. Nos. 2,329,059; 2,638,794; 2,830,466 and 3,791,238, wherein the differential gears are conventionally provided with fixing rings which are shiftable angularly between positions of frictional engagement and disengagement relatively to a central driving spider, or planet carrier, to thereby disengage temporarily a clutch associated with an axle shaft turning at a higher speed than the other axle shaft.
A further example of a differential gear is disclosed by U.S. Pat. No. 3,397,593 which has an elastic ring for centering a cam central member relatively to a driving spider. In this differential gear mechanism, the cam central member is centered on the driving spider by control cams arranged on either sides of the central cam.
The above-mentioned differential gear mechanisms, while performing in a generally satisfactory manner, still have too complex a construction, involve high-precision machining of the gears or cams, and are therefore exceedingly expensive.
More differential mechanisms are described in U.S. Pat. Nos. 1,414,126 and No. 1,498,279, which are incorporated hereto for reference.
The mechanism of the former of these references has the significant drawback of excluding the mechanical connection between the axle shafts and the drive input mechanism (bevel pinion and ring bevel gear) of the differential gear when no drive or direct resistance is applied between the said mechanism and the wheels on the corresponding axle. Such a situation may be encountered on a descent, for example, when the driving or resisting torques transferred between the axle shafts and the differential mechanism cancel out in consequence of the vehicle movement due to gravity, or with the vehicle at a standstill. In either case, the differential mechanism would enter the idling condition shown in FIG. 2 of the patent. This is a condition which may represent an evident danger where a brake device, for example, is arranged upstream of the differential mechanism to act on both axle shafts through the differential mechanism. In that case, any drive connection between the brake device and the axle wheels would be cut off. In addition, this known differential mechanism makes ample use of elastic members, which are potentially liable to break or yield, and consequently alter the operational characteristics, and in all cases constitute a costly structural complication.
The mechanism of the latter US reference has, on the other hand, a drawback in that it uses pawls for positive engagement of the axle shafts which are actuated by means of oscillating leaf springs. Once again, the leaf springs are liable to break or yield, and their oscillation, which results in either of the actuated pawls becoming engaged while the other is disengaged, may be hampered by any presence of deposits or metal matter in the socket for the pin around which the oscillation takes place. In addition, the pawls are a one-piece construction.